Civil Engineering Reference
In-Depth Information
Ta r
Wood tar gives a weak brown colour, due to the coal dust and pitch. Pigment
such as English red or ochre is stirred in to give beautiful and durable colours,
but it is impossible to paint other colours on top of a tarred wall. For tar stains
the binder can be supplemented with linseed oil or alkyde oil and thinned out
with organic solvents; fungicides can also be added.
Wood tar is extracted from coniferous and deciduous trees. Wood tar is usual-
ly rich in polycyclical aromatic hydrocarbons (PAHs). An exception is tar extract-
ed from beech. PAH substances such as benzoapyrene are carcinogenic and
mutagenic, so tar products should not be used indoors. When used outdoors, the
PAH substances will filter into the soil.
Re-use and recycling of painted products can be a problem. As waste, the
products should be deposited at special dumps.
Natural resins
Several different types of natural resins can be used for varnishing wood. To
make the resin more fluid, organic solvents can be added. A varnish layer of nat-
ural resin is about as vapour-proof as synthetic products, but it is less durable
and needs a longer drying time, and is also more expensive.
Cholophonium is extracted from the resin of pine trees after distilling veg-
etable turpentine oil, and consists mainly of abietic acid. This is seldom used as
the only resin in varnish mixes. It can be dissolved in alcohol or vegetable tur-
pentine. Copal, a fossil form of resin, is extracted in India, the Philippines,
Australia and Africa. Alcohol or vegetable turpentine are used as solvents.
Shellac comes from the Bengal fig tree (
Ficus bengalensis
) when it is attacked
by wood lice. Alcohol is used as a solvent. Dammar comes from special trees
in East India and Malaysia (
Dipterocarpaceae
). Alcohol or vegetable turpentine
can be used as solvents. Sanderac is drawn from the juniper gum tree
(
Callitris quadrivalis
) in Morocco. It dissolves in alcohol, turpentine, ether and
acetone.
Rubber mastic is extracted from the resin of the mastic tree (
Pistacia lentiscus
)
found on Mediterranean islands. It dissolves in alcohol or ether. Elimi gum is
resin extracted from amyris trees (
Burseraceae
) on the Philippines, Mauritius,
Mexico and Brazil. It dissolves in alcohol, petroleum or vegetable turpentine.
Acaroid resin is from the grass tree (
Xanthorrhoea australe
) in Australia. It dis-
solves in alcohol.
The products are mainly based on renewable resources, with the exception of
some types of solvent. During application, substances can be emitted by the
